All-solid-state secondary cell

ABSTRACT

An ion conductive glass ceramics having the formula Na 2 S—P 2 S 5 , wherein the Na 2 S in the ion conductive glass ceramics is contained in an amount of from 70 to 75 mole %, and wherein the ion conductive glass ceramics has a state where crystal parts are dispersed in the glass ingredient of an amorphous state and where the crystal parts contain tetragonal Na 3 PS 4 .

TECHNICAL FIELD

The present invention relates to an all-solid-state secondary cell(battery). More particularly, it relates to an all-solid-state secondarycell being at low cost and having high capacity where abundant sodiumresources are the background.

BACKGROUND ART

Since a lithium secondary cell has high voltage and high capacity, ithas been widely used as an electric source for mobile phones, digitalcameras, video cameras, notebook computers, electric cars, etc. Thelithium secondary cell which is commonly circulated uses a liquidelectrolyte where electrolytic salt as an electrolyte is dissolved in anon-aqueous solvent. Since the non-aqueous solvent contains muchcombustible solvent, there has been a demand for securing its safety.

Further, in recent years, demand for the lithium secondary cell has beenincreasing for storing the electric power in generating devices in cars(such as electric car and hybrid car), solar battery, wind powergeneration, etc. However, since the lithium secondary cell uses lithiumwherein the estimated deposit amount is small and the producing placesare unevenly distributed, there is an anxiety that the demand exceedsthe supply and there is also a problem of high cost.

For solving the above problem, as an all-solid-state secondary cellusing no solvent, a sodium-sulfur cell (NAS cell) which is a kind ofsodium secondary cell has been receiving public attention as a big-sizecell for storage of electric power.

Since the NAS cell is operated at the temperature of as high as notlower than 300° C., careful attention is needed for the handling ofsodium in a liquid state and there has been a problem in terms of itssafety.

Moreover, in the NAS cell, β-alumina is used as its sodiumion-conductive solid (electrolyte). β-Alumina shows a sodium ionconductivity of not less than 10⁻³ Scm⁻¹ at room temperature [Document:X. Lu et al., Journal of Power Sources, 195 (2010) 2431-2442]. However,for the production of β-alumina, burning at the temperature of as highas not lower than 1600° C. is necessary and there is a problem that itssolid interfacial adhesion to a positive electrode active material isdifficult.

Accordingly, in order to provide a highly safe all-solid-state secondarycell, there has been a demand for lowering the operating temperature toabout room temperature. In addition, a material which does not need theburning at high temperature and shows high conductivity as a moldedproduct of powder being produced merely by means of press is importantfor the interfacial constitution comprising a positive electrode and anelectrolyte for an all-solid-state secondary cell of a low-temperatureoperation type.

In view of the above, the inventors of the present invention haveproposed to use Na₂S—P₂S₅ glass ceramics as a solid electrolyte(Abstracts of Presentations at the 36th Symposium on Solid State Ionicsin Japan, (2010) page 120).

PRIOR ART DOCUMENT Non-Patent Document

Non-Patent Document 1: Abstracts of Presentations at the 36th Symposiumon Solid State Ionics in Japan, (2010) page 120.

SUMMARY OF THE INVENTION Problems That the Invention is to Solve

In the above document, the solid electrolyte having a sufficientlyconductive is obtained. However, since the sodium all-solid-statesecondary cell of a low-temperature operating type is a very newtechnique, there is still enough ground for consideration in selecting apositive electrode active material which is necessary for actuallyassembling into the cell.

In selecting the positive electrode active material, there are variousstandards. One of said standards is capacity of the cell. When capacityof the cell becomes large, it is possible to store the electric power inlarge amount even if the cell is small whereby its industrial value ishigh.

Accordingly, it is a problem to solve by the present invention toprovide a combination of the solid electrolyte made of glass ceramicsconstituted from Na₂S and sulfide with the positive electrode activematerial by which capacity of a cell can be enhanced in the sodiumall-solid-state secondary cell of a low-temperature operating type.

Means for Solving the Problems

As a result of earnest investigations, the inventors of the presentinvention found that, when Na₂S_(x) is used as the positive electrodeactive material, an all-solid-state secondary cell of high capacity canbe provided and they have achieved the present invention. Incidentally,so far as the present inventors are aware of, there is no documentreporting that the capacity is specifically enhanced by means of acombination of Na₂S_(x) as the positive electrode active material withglass ceramics constituted from a Na₂S and a sulfur compound as thesolid electrolyte.

Thus, in accordance with the present invention, there is provided anall-solid-state secondary cell comprising at least a positive electrode,a negative electrode and a solid electrolyte layer which is positionedbetween the positive electrode and the negative electrode, wherein:

-   -   the positive electrode contains an positive electrode active        material consisting of a Na₂S_(x) (x=1 to 8) and    -   the solid electrolyte layer contains an ion conductive glass        ceramics represented by a formula (I): Na₂S-M_(x)S_(y)        wherein M is selected from P, Si, Ge, B and Al; x and y each is        an integer giving a stoichiometric ratio depending upon the type        of M; and Na₂S is contained in an amount of more than 67 mole %        and less than 80 mole %.

Effect of the Invention

In accordance with the present invention, it is now possible to providea sodium all-solid-state secondary cell having high capacity of alow-temperature operating type which is not dependent on the resourceamount of lithium.

Further, as a result of any of or a combination of the followings, thesodium all-solid-state secondary cell of a low-temperature operatingtype having higher capacity can be provided:

-   -   Na₂S_(x) is Na₂S;    -   Na₂S-M_(x)S_(y) is Na₂S—P₂S₅;    -   Na₂S—P₂S₅ contains Na₂S in an amount of more than 67 mole % and        less than 80 mole %;    -   The ion conductive glass ceramics has a state where crystal        parts are dispersed in the glass ingredient of an amorphous        state;    -   The crystal parts are contained in an amount of 50% by weight or        more to the ion conductive glass ceramics as a whole; and    -   The positive electrode further contains Na₂S-M_(x)S_(y) to be        used for the solid electrolyte layer and the positive electrode        active material is contained in an amount of within a range of        10 to 90% by weight in the positive electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a XRD pattern of the glass in Example 1.

FIG. 2 is a DTA curve of the glass in Example 1.

FIG. 3 is a Raman spectrum of the glass in Example 1.

FIG. 4 is partial expansion of FIG. 3.

FIG. 5 is ³¹PMAS-NMR of the glass in Example 1.

FIGS. 6(a) to (d) are graphs showing the temperature dependency ofelectric conductivity of the glass and the glass ceramics in Example 1.

FIG. 7 is a graph showing the electric conductivity and the conductionactivation energy, under room temperature, of the glass and the glassceramics in Example 1.

FIG. 8 is a XRD pattern of the glass ceramics in Example 1.

FIG. 9 is ³¹PMAS-NMR of the glass ceramics in Example 1.

FIG. 10 is a charge-discharge curve of the all-solid-state secondarycell of Example 1.

FIG. 11 is a charge-discharge curve of the all-solid-state secondarycell of Comparative Example 1.

FIG. 12 is a charge-discharge curve of the all-solid-state secondarycell of Example 2.

MODE FOR CARRYING OUT THE INVENTION

The all-solid-state secondary cell of the present invention (sodiumall-solid-state secondary cell of a low-temperature operating type) isequipped with at least a positive electrode, a negative electrode and asolid electrolyte layer which is positioned between the positiveelectrode and the negative electrode. Here, “low temperature” means thetemperature where charge and discharge are possible and it is lower thanthe melting point of the components constituting the all-solid-statesecondary cell and, for example, it means the range of 0 to 100° C.Further, the term “all-solid-state secondary cell” means a secondarycell containing the electrolyte free of solvent in the cell.

(Positive Electrode)

The positive electrode contains a positive electrode active materialcomprising Na₂S_(x) (x=1 to 8). This positive electrode active materialis common to a substance which is able to be contained in the solidelectrolyte layer. Therefore, between the positive electrode and thesolid electrolyte layer, formation of non-contacting interface of themcan be prevented and a conductive path for the transfer of Na in thepositive electrode active material to the solid electrolyte layer duringthe charge can be easily formed. There is an additional advantage thatthe amount of Na to be transferred to the solid electrolyte layer can bemade abundant.

As to Na₂S_(x), a sulfide such as Na₂S, Na₂S₂, Na₂S₃, Na₂S₄ or Na₂S₈ maybe exemplified. As to the sulfides as such, either one of them or amixture thereof may be used. Among those sulfides, Na₂S, Na₂S₂ and Na₂S₄are preferred and Na₂S is more preferred.

The positive electrode active material may contain an active materialother than Na₂S. Examples thereof are various transition metal oxidessuch as Na_(0.44)MnO₂, NaNi_(0.5)Mn_(0.5)O₂, FeS, TiS₂, Mo₆S₈, MoS₂,NaCoO₂, NaFeO₂, NaCrO₂, Na₃V₂(PO₄)₃ and NaMn₂O₄.

The positive electrode may be composed of only the positive electrodeactive material or may be mixed with a binder, a conductive agent, anelectrolyte, etc. Content of Na₂S in the positive electrode is preferredto be within a range of 10 to 90% by weight. When the content is lessthan 10% by weight, it is hard to obtain the all-solid-state secondarycell of high capacity. When the content is more than 90% by weight,amount of Na which does not contribute in a cell reaction becomes highwhereby the utilizing rate of the positive electrode active material maylower. The content may, for example, be 10% by weight, 20% by weight,30% by weight, 40% by weight, 50% by weight, 60% by weight, 70% byweight, 80% by weight or 90% by weight. More preferred content is withina range of 25 to 75% by weight.

As to the binder, there may be exemplified polyvinylidene fluoride,polytetrafluroethylene, polyvinyl alcohol, polyvinyl acetate, polymethylmethacrylate and polyethylene.

As to the conductive agent, there may be exemplified natural graphite,artificial graphite, acetylene black, Ketjenblack, Denka black, carbonblack and vapor-grown carbon fiber (VGCF).

As to the electrolyte, there may be exemplified an electrolyte used forthe solid electrolyte layer which will be mentioned later.

The positive electrode may be prepared as pellets by, for example,mixing the positive electrode active material optionally with thebinder, the conductive agent, the electrolyte, etc. followed by pressingthe resulting mixture. Here, in mixing the conductive agent and theelectrolyte with the positive electrode active material, there is noparticular limitation for a mixing means. Thus, there may be exemplifieda mixing using a mortar and a mixing by means of mechanical milling.Particularly in mixing the electrolyte with other ingredient, it ispreferred to be conducted by means of a mechanical milling by which itis possible to mix more uniformly.

The positive electrode may be formed on a collector such as stainlesssteel, Al or Cu.

(Solid Electrolyte Layer)

The solid electrolyte layer contains an ion-conductive glass ceramicsrepresented by the formula (I): Na₂S-M_(x)S_(y), wherein M is selectedfrom P, Si, Ge, B and Al; x and y each is an integer giving astoichiometric ratio depending upon the type of M; and Na₂S is containedin an amount of more than 67 mole % to less than 80 mole %. Specificexamples of the glass ceramics are Na₂S—P₂S₅, Na₂S—SiS₂, Na₂S—GeS₂,Na₂S—B₂S₃ and Na₂S—Al₂S₃. Among them, Na₂S—P₂S₅ is particularlypreferred. Further, another ion-conductive material such as NaI orNa₃PO₄ may also be added thereto. The amount of the ion-conductive glassceramics in the solid electrolyte layer is preferably 90% by weight ormore and, more preferably, the total amount.

In addition, Na₂S-M_(x)S_(y) contains Na₂S in an amount of more than 67mole % and less than 80 mole %. When the amount is within this range,ion conductivity can be enhanced as compared with the correspondingglass. The rate of Na₂S may, for example, be 79 mole %, 70 mole %, 60mole %, 50 mole %, 40 mole %, 30 mole %, 20 mole % or 10 mole %. It ismore preferred to contain more than 70 mole % and less than 80 mole % ofNa₂S and it is still more preferred to contain 73 to 77 mole % of Na₂S.

The ion-conductive glass ceramics may be in such a state where crystalpart is dispersed in a glass ingredient of an amorphous state. The rateof the crystal part to the whole glass ceramics is preferably 50% byweight or more and, more preferably, 80% by weight or more. This ratemay, for example, be by 50% by weight, 60% by weight, 70% by weight, 80%by weight or 90% by weight. The rate of the crystal part can be measuredby means of a solid NMR.

Incidentally, in the glass ceramics, it is preferred that there is noglass transition point which was present in the corresponding glass.

Thickness of the solid electrolyte layer is preferably 1 to 1000 μm and,more preferably, 1 to 200 μm. This thickness may, for example, be 1 μm,100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μmor 1000 μm. The solid electrolyte layer may, for example, be prepared inpellets by pressing the material thereof.

A process for producing the above ion-conductive glass ceramicsincludes, for example, the following step.

(i) a step where a material mixture containing Na₂S and M_(x)S_(y) in apredetermined rate for giving the formula (I): Na₂S-M_(x)S_(y) issubjected to a mechanical milling treatment to give glass; and

(ii) a step where glass is converted to the ion-conductive glassceramics by subjecting the glass to a heat treatment at the temperatureof a glass transition point or higher.

(1) Step (i)

In the mechanical milling treatment in the step (i), there is noparticular limitation for its treating device and treating condition sofar as the materials can be well mixed and made to react.

As to a treating device, a ball mill may be usually used. The ball millis preferred since a big mechanical energy can be obtained thereby.Among the ball mill, a preferred one is a planet-type ball mill since apot rotates on its axis while a stand plate revolves around whereby highimpact energy can be efficiently generated.

A treating condition may be appropriately set depending upon thetreating device used therefor. For example, when a ball mill is used,the materials can be more uniformly mixed and made to react if therevolving velocity is higher and/or the treating time is longer.Incidentally, the term reading “and/or” means A, B or, A and B whenexpressed as A and/or B. To be more specific, when the planet-type ballmill is used, such a condition where revolving velocity of 50 to 600revolutions per minute, treating time of 0.1 to 50 hour(s) and 1 to 100kWh for 1 kg of a material mixture may be exemplified. The revolvingvelocity may, for example, be 50 rpm, 100 rpm, 200 rpm, 300 rpm, 400rpm, 500 rpm or 600 rpm. The treating time may, for example, be 0.1hour, 10 hours, 20 hours, 30 hours, 40 hours or 50 hours. Electric powerfor the treatment may, for example, be 6 kWh/1 kg of material mixture,10 kWh/1 kg of material mixture, 20 kWh/1 kg of material mixture, 30kWh/1 kg of material mixture, 40 kWh/1 kg of material mixture, 50 kWh/1kg of material mixture or 60 kWh/1 kg of material mixture. Morepreferred treating condition may be revolving velocity of 200 to 500rpm, treating time of 1 to 20 hour(s) and 6 to 50 kWh for 1 kg of thematerial mixture.

(2) Step (ii)

The glass obtained in the above step (i) is subjected to a heattreatment to convert to the ion-conductive glass ceramics. This heatingtreatment is carried out at a temperature which is a glass transitionpoint or higher.

The glass transition point (T_(g)) varies depending upon the ratio ofNa₂S to M_(x)S_(y) and, in the case of Na₂S—P₂S₅ for example, it iswithin a range of 180 to 200° C. T_(g) may, for example, be 180° C.,185° C., 190° C., 195° C. or 200° C. The first crystallizing temperature(T_(c)) is within a range of 190 to 240° C. T_(c) may, for example, be190° C., 200° C., 210° C., 220° C., 230° C. or 240° C. Although theupper limit of the temperature for the heat treatment is notparticularly limited, it is usually the first crystallizing temperature+100° C.

The heat treatment time is the time by which the glass can be convertedto the ion-conductive glass ceramics and, when the heat treatmenttemperature is high or low, the time becomes short or long,respectively. The time for heat treatment is usually within a range of0.1 to 10 hour(s). The time for heat treatment may, for example, be 0.1hour, 3 hours, 5 hours, 7 hours, 9 hours or 10 hours.

There is no particular limitation for the negative electrode. Thenegative electrode may comprise a negative electrode active materialonly or may be mixed with a binder, a conductive agent, an electrolyte,etc.

As to the negative electrode active material, there may be exemplifiedmetal (such as Na, In or Sn), Na alloy, graphite, hard carbon andvarious transition metal oxides (such as Li_(4/3)Ti_(5/3)O₄, Na₃V₂(PO₄)₃and SnO). A range of the rate of the negative electrode active materialin the negative electrode may be set at about the same range of the rateof the positive electrode active material in the positive electrode.

As to the binder, the conductive agent and the electrolyte, any of themlisted in the above column for the positive electrode may be used.

The negative electrode may be obtained in pellets by, for example,mixing the negative electrode active material with the binder, theconductive agent, the electrolyte, etc. followed by pressing theresulting mixture. When a metal sheet (foil) is used as the negativeelectrode active material comprising metal or alloy thereof, it may beused just as it is.

The negative electrode may be formed on a collector such as stainlesssteel, Al or Cu.

(Process for Production of all-Solid-Phase Secondary Cell)

The all-solid-phase secondary cell may, for example, be produced bylayering the positive electrode, the solid electrolyte layer and thenegative electrode followed by pressing.

EXAMPLES

Although the present invention will be more specifically illustrated byway of the following Examples, it is not limited by those Examples atall.

Example 1

(Production of Solid Electrolyte Layer)

Step (i): Mechanical Milling Treatment

Na₂S (manufactured by Aldrich; purity: 99%) and P₂S₅ (manufactured byAldrich; purity: 99%) in a mole ratio of 67:33, 70:30, 75:25 or 80:20were poured into a planet-type ball mill. After pouring, a mechanicalmilling treatment was carried out to give 67Na₂S-33P₂S₅, 70Na₂S-30P₂S₅,75Na₂S-25P₂S₅ or 80Na₂S-20P₂S₅, respectively.

As to the planet-type ball mill, Pulverisette P-7 manufactured byFritsch was used where pot and ball were made of ZrO₂ and a mill where500 balls of 4 mm diameter were placed in a 45-ml pot was used. Themechanical milling treatment was carried out for 20 hours at therevolving velocity of 510 rpm and room temperature in a glove box of adry nitrogen atmosphere.

In the meanwhile, the above production process was in accordance withthe description in “Experimental” in Akitoshi Hayashi, et al., Journalof Non-Crystalline Solids, 356 (2010), pages 2670 to 2673.

When 80 mg of each of the above four kinds of Na₂S—P₂S₅ was pressed(pressure: 370 MPa/cm²), there were produced pellets of 10 mm diameterand about 1 mm thickness.

With regard to the resulting glass, its XRD pattern is shown in FIG. 1,DTA curve is shown in FIG. 2, Raman spectrum is shown in FIG. 3, partialexpansion of FIG. 3 is shown in FIG. 4 and ³¹PMAS-NMR is shown in FIG.5.

From FIG. 1, it is shown that an amorphous material was prepared in thecase of 67Na₂S-33P₂S₅, 70Na₂S-30P₂S₅ and 75Na₂S-25P₂S₅ and that, in thecase of 80Na₂S-20P₂S₅, a part of Na₂S remained in addition to theamorphous thing.

From FIG. 2, the glass transition point was confirmed in all of67Na₂S-33P₂S₅, 70Na₂S-30P₂S₅, 75Na₂S-25P₂S₅ and 80Na₂S-20P₂S₅ whereby itis noted that those amorphous materials are in a state of glass.Incidentally, the glass transition point is between 180° C. and 200° C.

From FIG. 3 and FIG. 4, peaks derived from P₂S₇ ⁴⁻ were mostly noted inthe case of 67Na₂S-33P₂S₅. In the case of 70Na₂S-30P₂S₅, peaks derivedfrom P₂S₇ ⁴⁻ decreased as the rate of Na₂S increased and, in place ofsuch peaks, the peaks derived from PS₄ ³⁻ increased; and, in the casesof 75Na₂S-25P₂S₅ and 80Na₂S-20P₂S₅, peaks derived from PS4³⁻ were mostlynoted. Incidentally, the fact that assignments of peaks in FIG. 3 andFIG. 4 were derived from PS₄ ³⁻, P₂S₇ ⁴⁻ and P₂S₆ ⁴⁻ was analogized fromthe data in an Li₂S—P₂S₅ system since no data in an Na₂S—P₂S₅ wereavailable. To be more specific, PS₄ ³⁻, P₂S₇ ⁴⁻ and P₂S₆ ⁴⁻ wereanalogized as 419 cm⁻¹, 406 cm⁻¹ and 382 cm⁻¹, respectively.

In FIG. 5, the same tendency as in FIGS. 3 and 4 was noted as well.

Step (ii): Heat Treatment

Pellets comprising each of the above-mentioned four kinds of glass wereheated starting from room temperature (25° C.) to 280° C. (not lowerthan crystallizing temperature) so that the glass was made into theglass ceramics. Further, after reaching 280° C., the glass ceramics wascooled down to room temperature to give the solid electrolyte layer (70mg) of 10 mm diameter and 1 mm thickness. During this heating andcooling cycle, electric conductivity of the pellet was measured everyabout 15° C. Results of the measurement are shown in FIGS. 6(a) to (d).In the drawings, black dots show the glass ceramics and open circlesshow the glass.

From FIG. 6(a), it is shown that, in 67Na₂S-33P₂S₅, there is almost nodifference in terms of electric conductivity between the glass state andthe glass ceramics state. From FIGS. 6(b) to (d), it is shown that, in70Na₂S-30P₂S₅, 75Na₂S-25P₂S₅ and 80Na₂S-20P₂S₅, differences wereresulted in terms of electric conductivity between the glass state andthe glass ceramics state. Particularly in the former two cases, theglass ceramics state shows higher electric conductivity than the glassstate.

In addition, the result where activation energies (Ea) of conductance ofglass and glass ceramics were measured is shown in Table 1 together withthe data of electric conductivity at room temperature. In Table 1, Gmeans the glass and GC means the glass ceramics. Further, the result ofTable 1 is collectively shown in FIG. 7. In FIG. 7, black dots and blacktriangles stand for the glass ceramics while open circles and opentriangles stand for the glass.

TABLE 1 conduction electric activation conductivity energy mole ratio %Scm⁻¹ kJmol⁻¹ of Na₂S G GC G GC 67 2.1 × 10⁻⁶ 1.8 × 10⁻⁶ 47 51 70 2.8 ×10⁻⁶ 4.7 × 10⁻⁵ 48 31 75 3.7 × 10⁻⁶ 2.6 × 10⁻⁴ 53 25 80 9.9 × 10⁻⁶ 1.9 ×10⁻⁶ 41 42

From FIG. 7 and Table 1, it is noted that the electric conductivity andthe activation energy of conduction are different between the glassstate and the glass ceramics state.

XRD patterns and ³¹PMAS-NMR of 67Na₂S-33P₂S₅, 70Na₂S-30P₂S₅,75Na₂S-25P₂S₅ and 80Na₂S-20P₂S₅ after the heat treatment are shown inFIG. 8 and FIG. 9, respectively. FIG. 8 also shows XRD pattern of Na₃PS₄crystals (tetragonal) published in the following document A.

Document A: M. Jansen, et al., Journal of Solid State Chemistry,92(1992), 110.

It is noted from FIG. 8 that four kinds of Na₂S—P₂S₅ are in the glassceramics state because of the presence of peaks derived from crystalstructure as compared with FIG. 1. It is also noted from FIG. 8 that,although the same peak pattern as Na₃PS₄ crystals is available in80Na₂S-20P₂S₅, separation of crystals which are different from Na₃PS₄crystals is predicted since the presence of a pattern which is differentfrom Na₃PS₄ crystals is observed as mole % of Na₂S becomes small.Particularly in the case of 75Na₂S-25P₂S₅, although a pattern is notedat the 2θ position similar to the pattern of tetragonal Na₃PS₄, nofission of the peak is noted whereby it is likely that cubic Na₃PS₄ ispresent. Further, in 67Na₂S-33P₂S₅, no identification of crystals ispossible and separation of unknown crystals is predicted. It is alsofound that, in 70Na₂S-30P₂S₅, a pattern which is the sum of the patternsof 67Na₂S-33P₂S₅ and 75Na₂S-25P₂S₅ is resulted.

From FIG. 9, peaks derived from PS₄ ³⁻ are mostly noted in 75Na₂S-25P₂S₅and 80Na₂S-20P₂S₅ and peaks derived from P₂S₇ ⁴⁻ are mostly noted in67Na₂S-33P₂S₅. In 70Na₂S-30P₂S₅, both peaks derived from PS₄ ³⁻ and P₂S₇⁴⁻ are noted.

(Production of Positive Electrode)

Na₂S as a positive electrode active material, acetylene black (HS-100manufactured by Denki Kagaku Kogyo of Japan) as a conductive agent andthe glass ceramics made by the above 75Na₂S-25P₂S₅ as a solidelectrolyte were weighed in the ratio of 25:25:50% by weight (totalweight: 15.5 mg).

The positive electrode active material and a conductive agent were mixedusing a mortar for 10 minutes. After that, the resulting two-ingredientmixture was subjected to the mechanical milling treatment the same asabove together with the solid electrolyte. The resultingthree-ingredient mixture was pressed to give a positive electrode of 10mm diameter and 100 μm thickness.

(Negative Electrode)

As to a negative electrode, an In foil of 9 mm diameter and 0.1 mmthickness was used.

(Production of All-Solid-State Secondary Cell)

The resulting positive electrode, solid electrolyte (75Na₂S-25P₂S₅) andnegative electrode were layered in this order and the resulting layeredproduct was sandwiched using stainless steels as a positive electrodecollector and a negative electrode collector followed by pressing togive an all-solid-state secondary cell. The resulting all-solid-statesecondary cell was subjected to the following charge-discharge test.

Charge-discharge condition: under room temperature, current density0.013 mA/cm² and potential range 0 to 3 V (vs. Na)

A charge-discharge curve of the resulting all-solid-state secondary cellis shown in FIG. 10. From FIG. 10, the resulting all-solid-statesecondary cell showed the capacity of about 200 mAh/g in the firstcharge. Even in the second charge and thereafter, the capacity of about80 mAh/g was noted and the resulting all-solid-state secondary cell hada sufficient capacity.

Comparative Example 1

Na_(0.44)MnO₂ as a positive electrode active material, Glass ceramicsmade by 75Na₂S-25P₂S₅ of Example 1 as an electrolyte and acetylene blackas a conductive agent were weighed in the ratio by weight of 40:60:6(total weight: 15 mg) followed by mixing and pressing to give a positiveelectrode.

Na_(0.44)MnO₂ was prepared as follows. Firstly, Na₂CO₄ and Mn₂O₃ wereweighed in a mole ratio of 0.55:1. Na₂CO₄ and Mn₂O₃ were mixed in amortar for 30 minutes. The mixture was pressed to make into pellets andburned at 800° C. for 12 hours. The resulting burned product wassubjected to the same mechanical milling treatment as in Example 1 togive Na_(0.44)MnO₂.

As to a solid electrolyte layer, the same one as in Example 1 was used.

As to a negative electrode, such a one where metal sodium wasprecipitated onto stainless steel as a collector during the initialcharge stage was used.

Production of an all-solid-state cell was conducted in the same manneras in Example 1 except the use of the above positive electrode, solidelectrolyte layer and negative electrode.

A charge-discharge curve of the resulting all-solid-state secondarystate is shown in FIG. 11. From FIG. 11, the resulting all-solid-statesecondary cell showed the capacity of about 36 mAh/g in the first chargeand, as compared with the cell of Example 1, the capacity was apparentlylow.

Example 2

Na₂S as a positive electrode active material, acetylene black (HS-100manufactured by Denki Kagaku Kogyo) as a conductive agent and Glassceramics made by the above 75Na₂S-25P₂S₅ as a solid electrolyte wereweighed in the ratio of 25:25:50% by weight (total weight: 4.8 mg).

The positive electrode active material was subjected to a mechanicalmilling treatment for 10 hours at the revolving velocity of 230 rpm indehydrated toluene (The device used for the treatment was the same asthat in Example 1). After the conductive agent was added to the positiveelectrode active material treated as such, the mixture was subjected toa mechanical milling treatment for 10 hours at the revolving velocity of370 rpm. The resulting two-ingredient mixture was subjected to amechanical milling together with the solid electrolyte for 30 minutes atthe revolving velocity of 300 rpm. The resulting three-ingredientmixture was pressed to give a positive electrode.

(Negative Electrode)

Sn as an negative electrode active material and Glass ceramics made bythe above 75Na₂S-25P₂S₅ as a solid electrolyte were weighed in the ratioof 70:30% by weight (total weight: 42.1 mg).

The negative electrode active material and the conductive agent weremixed for 10 minutes in a mortar. The resulting two-ingredient mixturewas pressed to give a negative electrode.

(Production of All-Solid-State Secondary Cell)

An all-solid-state secondary cell was produced from the resultingpositive electrode, solid electrolyte layer 75Na₂S-25P₂S₅) and negativeelectrode according to the same manner as in Example 1. The resultingall-solid-stat secondary cell was subjected to the followingcharge-discharge test.

Charging-discharging condition: under room temperature, current density0.013 mA/cm² and potential range 0 to 3 V (vs. Na—Sn)

A charge-discharge curve of the resulting all-solid-state secondary cellis shown in FIG. 12. From FIG. 12, the resulting all-solid-statesecondary cell showed the capacity of about 750 mAh/g in the firstcharge. Even in the second charge and thereafter, the capacity of about300 mAh/g was noted and the resulting all-solid-state secondary cell hada sufficient capacity.

What is claimed is:
 1. An ion conductive glass ceramics having a formulaNa₂S—P₂S₅, wherein the Na₂S in the ion conductive glass ceramics iscontained in an amount of from 70 to 75 mole %, and wherein the ionconductive glass ceramics has a state where crystal parts are dispersedin a glass ingredient of an amorphous state and where the crystal partscontain tetragonal Na₃PS₄.
 2. The ion conductive glass ceramics of claim1, wherein the ion conductive glass ceramics is contained in a solidelectrolyte layer of an all-solid-state secondary cell.
 3. The ionconductive glass ceramics of claim 1, wherein the crystal parts arecontained in an amount of 50% by weight or more of the ion conductiveglass ceramics.
 4. An all-solid-state secondary cell comprising at leasta positive electrode, a negative electrode and a solid electrolytelayer, wherein the solid electrolyte layer contains the ion conductiveglass ceramics of claim 1.